1. Field of the Invention
The present invention relates to an electrode for electrochemical measurement used in sensors for detecting specific substances.
2. Description of the Related Art
Conventionally, attempts have been made to use carbon nanotubes as electronic devices because of their semiconductive characteristics.
The advantage of using carbon nanotubes as electronic devices is their extremely high conductivity. Their small diameters of 1 to 20 nm or so are also suitable to be used as devices and electrodes in micro circuits.
On the other hand, from the viewpoint of the application to the medical field, biosensors for detecting trace amounts of biological substances are being heavily studied and developed. Detecting methods in the actual use may be classified into these three methods:    (1) electrochemical method;    (2) enzymatic method; and    (3) color reaction method.
Among these, the electrochemical method is the most widespread detecting method. Substances have inherent oxidation-reduction potentials, and by applying a specific potential, electrons may be pulled out of (oxidation) or injected into (reduction) a substance. Therefore, electron transfer involved in the oxidation-reduction reactions may be measured by cyclic voltammetry or the like so as to find the amount of the target substance from the current value at a given potential.
In the electrochemical method, micro electrodes are used to improve detecting sensitivity. The micro electrodes refer to electrodes having sizes of a μm level or smaller, while the electrodes generally used for electrochemical measurement have sizes of several millimeters to several centimeters (See, for example, S. Pons and M. Fleischmann, Analytical Chemistry, 1987, vol. 59, page 1391A). Use of such micro electrodes provides the following advantages (1) to (4):    (1) contribution of charging currents causing noises may be reduced;    (2) potentials may be swept at a high speed;    (3) influence of substance dispersion may be reduced; and    (4) highly sensitive measurement may be attained.
Owing to these advantages, use of micro electrodes has been widespread in detecting trace amounts of samples by the electrochemical method, and recently, higher sensitivity is desired.
On the other hand, the enzymatic method is a method for detecting a target substance electrochemically by using electrodes having an enzyme fixed on their surfaces. Enzymes have the feature of being capable of selectively detecting the target substance at comparatively high sensitivity from a mixture because they react specifically with the target substance. So far, glucose sensors (diabetes testing), uric acid sensors (gout testing), and urea sensors (kidney function testing) are already in actual use in the medical field. However, there is a problem that the enzyme is difficult to handle because it is instable and must be stored in a special circumstance to maintain its activity.
The color reaction method is a method for detecting a biological substance by measuring the ultraviolet-visible absorption spectrum using a sample which develops color when reacted with the target substance, and by finding its absorbance. However, the detecting sensitivity in the absorbance measurement is in proportion to the light path length, whereby a large number of sample solutions are needed to enhance sensitivity. Thus, the method has a problem of inability to be applied for detecting trace amounts of samples.
Sensors utilizing the electrochemical detecting method include: sensors for detecting a specific substance by using ion sensitive field-effect transistors (See, for example, Japanese Patent Application Laid-open (JP-A) No. 03-272449); sensors for detecting hydrogen peroxide by using an electrode with ferrocene fixed on its surface (See, for example, C. Padeste et. al, “Ferrocene-avidinconjugates for bioelelectochemical applications,” Biosensors & Bioelectronics, 2000, volume 15, pp. 431-8); and sensors for detecting a specific substance by using a carbon nanotube as an electrode and making use of changes in electric properties of the carbon nanotube by outer stimulations (See, for example, JP-A No. 2003-227808). However, these sensors are unsatisfactory in terms of the aforementioned viewpoint, and improvements are still demanded.